Noble metal support

ABSTRACT

A noble metal-supported article which comprises a carrier and a palladium-containing metal component supported on the carrier, which article has (A) a layer in which substantially no palladium is supported in the interior of the carrier and (B) a layer in which palladium is supported in the region from the outer surface to a depth of less than 100 μm of the carrier.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/JP97/04624 which has an Internationalfiling date of Dec. 16, 1997, which designated the United States ofAmerica.

DESCRIPTION

1. Technical Field

This invention relates to a metal-supported article in which apalladium-containing metal component is supported on a carrier, aprocess for producing the above metal-supported article and a catalystfor chemical reaction composed of the above metal-supported article.

2. Background Art

Palladium or a palladium metal compound has been widely used as acatalyst in industry for many years.

The term “palladium metal compound” used herein is meant to includepalladium intermetallic compounds in which the lattice of palladium isreplaced with diverse metal and palladium alloys wherein the palladiumand diverse metal form a solid solution.

Palladium has been used alone as a catalyst; however, in general,palladium is used in the form of a dispersion on a carrier becausepalladium is expensive and in view of the purpose of realizing a highreactivity. In chemical industry processes, a palladium-supportedcatalyst has been widely used in various reactions such as oxidationreactions, reduction reactions, hydrogenation reactions and the like andin catalysts for purging an automobile exhaust or the like. Palladium issupported on various carriers depending upon the reactioncharacteristics and uses. With a catalyst in which a noble metalcomponent is uniformly supported even in the interior of the carrier,the diffusion of reactants and reaction product can be prevented by theresistance of fine pores in the interior of the carrier. In this case,it follows that the catalyst components supported in the interior of thecarrier are not effectively utilized. Moreover, in some kinds ofreactions, some types of reactors and the like, the catalyst componentssupported on the particle surface of a carrier are deactivated becausetheir active sites are covered as a result of adsorption of reactionby-products, accumulation of poisoned substances and the like, or thecarrier particles are cut by abrasion due to collision of the carrierparticles with one another or with wall surfaces or the like. As aresult, the active components of the catalyst are lost, and the catalystis deactivated. Therefore, for the purpose of prolonging the life of thecatalyst while maintaining its reaction activity, it has been proposedfor a long time that the active components of the catalyst be supportedslightly inside the carrier surface without being distributed on theouter surface of the carrier. As the catalyst preparation method asmentioned above, there has been known an impregnation method, anion-exchange method, a chemical reaction method (for example, solublepalladium is insolubilized by reaction and deposited on a carrier) orthe like.

An example in which the distribution of a palladium component on acarrier has been changed is reported in the Journal of Chemical Societyof Japan (4), pp. 261-268 (1990). It is reported therein that whenspherical alumina having a particle diameter of 3 mm (3,000 μm) is usedas a carrier and palladium dissolved in hydrochloric acid is allowed tobe adsorbed thereon by ion exchange, the palladium distribution and theamount of palladium supported are varied with adsorption time, forexample, after one minute, after 25 minutes, after one hour, after 12hours and after 20 hours, and the adsorption reaches approximatelyequilibrium in about one hour. However, even when the adsorption time isone minute, substantially no palladium is distributed from the outersurface of the carrier to a depth of about 300 μm and palladium isdistributed in the form of a belt in a width of about 150 μm from thesaid about 300-μm depth in the direction of the carrier interior.Furthermore, the amount of palladium supported is increased with thelapse of time and palladium is uniformly distributed throughout thecarrier after at least one hour. In said publication, there is nostatement of a case of within one minute; however, when it is taken intoconsideration that after one minute, substantially no palladium ispresent in the area from the outer surface to a depth of about 300 μm ofcarrier and palladium is distributed in the form of a belt in a width ofabout 150 μm from said 300-μm depth in the direction of the carrierinterior, it is actually difficult to control the distribution by thismethod so that the palladium is allowed to be substantially presentwithin the very narrow range from the outer surface to a depth of 100 μmof carrier. Also, an example of the preparation of a catalyst in whichsilica gel is used as a carrier and palladium ions and ammonium ions areallowed to coexist is known, but in this case, palladium is uniformlysupported in the carrier particles.

No palladium-supported article in which palladium is present only in thevery narrow range from the outer surface to a depth of 100 μm in thecarrier has been known.

DISCLOSURE OF INVENTION

This invention aims at providing a supported article in which thedistribution of palladium in a carrier is controlled and Pd is allowedto be selectively present in a specific range in order to effectivelyutilize the noble metal Pd supported on a carrier. This invention alsoprovides a highly active palladium-supported article by allowingpalladium to be present in the vicinity of the surface of a carrier butnot to be present in the interior of the carrier. In addition, thisinvention is directed to a noble metal-supported composition having along catalyst life by further providing a layer in which no palladium ispresent as the outermost layer when accumulation of substances whichwould poison the catalyst and the loss of the active components due toabrasion may be encountered.

The present inventors have conducted research on supported catalysts andthe reaction characteristics thereof based on many reaction examples. Inmany cases in which expensive noble metal components such as palladiumand the like are used as the catalyst components, it is essential tosupport the catalyst components on a carrier. The present inventors havefound that it is effective to support palladium in such a way that thereactants can react instantly. That is to say, when the diffusion rateof the reactants in the fine pores of the carrier affects the reactionrate, the inventors have found that it is effective to provide acatalyst wherein the distribution of a noble metal component, such aspalladium or the like is as close as possible to the outer surface ofthe carrier, and no noble metal component is present in the centerportion of the carrier where the influence of the diffusion is great. Inaddition, in a palladium-containing metal-supported article effectivefor heavy oil reformation, reactions is which the accumulation ofpoisoned substances takes place such as in a combustion catalyst or thelike or in the case in which abrasion of catalyst particles is possibleas in a fluidized bed, a bubble column, a stirring type reactor or thelike, the inventors have found that the durability of the catalyst canbe greatly improved without causing a reduction of reaction activity bysupporting a noble metal component wherein a thin layer free frompalladium is used as the outermost layer with the catalyst particle. Ifthe durability can be improved without reducing the reaction activity,the catalyst life could be greatly prolonged and the economics could beimproved. Heretofore, no technique for obtaining a metal-supportedarticle in which a noble metal component is distributed in a very thinlayer of less than 100 microns or technique for obtaining ametal-supported article which has a very thin layer substantially freefrom palladium-containing metal on the surface without reducing thereaction activity and wherein the palladium is supported in the surfacelayer having a thickness of less than 100 microns but is not present inthe interior have bee known in the art.

Thus, the present invention is directed to a method for preciselycontrolling the vicinity of the surface of a carrier for palladium- or apalladium metal compound-supported catalyst.

The invention comprises the following:

(1) A noble metal-supported article which comprises a carrier and apalladium-containing metal component supported on the carrier and whicharticle has (A) a layer in which substantially no palladium is supportedin the interior of the carrier and (B) a layer in which palladium issupported in the region from the outer surface to a depth of less than100 μm of the carrier.

(2) The noble metal-supported article according to (1) above, which has(A1) a layer in which substantially no palladium is supported on theouter surface of the carrier and has (A2) a layer in which substantiallyno palladium is supported in the center portion of the carrier.

(3) The noble metal-supported article according to (1) or (2) above,wherein the thickness or particle diameter of the carrier is not lessthan 200 μm.

(4) The noble metal-supported article according to (1) or (2) above,wherein the thickness or particle diameter of the carrier is less than200 μm and which article has the layer (B) in which palladium issupported in the region from the outer surface to a depth of less than80 μm of the carrier.

(5) The noble metal-supported article according to any one of (1) to (4)above, wherein the palladium-containing metal component is palladium orthe palladium metal compound.

(6) The noble metal-supported article according to (5) above, whereinthe palladium metal compound is a palladium-lead intermetallic compound.

(7) The noble metal-supported article according to (5) above, whereinthe palladium metal compound is a palladium-bismuth intermetalliccompound.

(8) A process for producing the noble metal-supported article accordingto any one of (1) and (3) to (7) above, which comprises instantlyintroducing a dried and/or calcined carrier containing a basic metalsalt component of at least one metal selected from the group consistingof alkali metals, alkaline earth metals and rare earth metals into apalladium compound solution maintained at a temperature of not less than70° C.

(9) A process for producing the noble metal-supported article accordingto any one of (2) to (7) above, which comprises instantly introducing adried and/or calcined carrier containing a basic metal salt component ofat least one metal selected from the group consisting of alkali metals,alkaline earth metals and rare earth metals into an aluminum compoundsolution maintained at a temperature of not less than 70° C. andthereafter momentarily introducing the resulting mixture into apalladium compound solution, or alternatively momentarily introducingthe above dried and/or calcined carrier into a solution of a mixture ofan aluminum compound and a palladium compound.

(10) A process for producing the noble metal-supported article accordingto any one of (1) and (3) to (7) above, which comprises dispersing adried and/or calcined carrier containing a basic metal salt component ofat least one metal selected from the group consisting of alkali metals,alkaline earth metals and rare earth metals in a solution containing asalt of at least one metal selected from the group consisting of alkalimetals, alkaline earth metals and rare earth metals and then contactingthe resulting dispersion with a palladium compound solution at atemperature of not less than 70° C.

(11) A process for producing the noble metal-supported article accordingto any one of (1) to (7) above, which comprises dispersing a driedand/or calcined carrier containing a basic metal salt component of atleast one metal selected from the group consisting of alkali metals,alkaline earth metals and rare earth metals in a solution containing asalt of at least one metal selected from the group consisting of alkalimetals, alkaline earth metals and rare earth metals and then contactingthe resulting dispersion firstly with an aluminum compound solution at atemperature of not less than 70° C. and secondly with a palladiumcompound solution at a temperature of not less than 70° C., oralternatively contacting the above dispersion with a solution of amixture of a palladium compound and an aluminum compound at atemperature of not less than 70° C.

(12) A process for producing an acrylic acid ester and/or a methacrylicacid ester, which comprises reacting an alcohol with acrolein and/ormethacrolein in the presence of oxygen using the noble metal-supportedarticle according to any one of (1) to (7) above as a catalyst.

Since in the noble metal-supported article of this invention, palladiumis allowed to be present in a region effective for the reaction in thecarrier, the catalytic activity is high and the expensive noble metalcan be utilized effectively. In addition, by providing apalladium-unsupported layer as the outermost layer, it becomes possibleto utilize the noble metal-supported article in many reactions as acatalyst which has a long life and is highly resistive to poisoning andabrasion. Moreover, by providing a palladium component-free layercontrolled to a very thin thickness on the surface of the carrier, agreat reduction of catalytic activity can be prevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the results of line analysis of the particle section of thepalladium-supported catalyst in Example 1 using an X-ray microprobe.

FIG. 2 shows the results of line analysis of the particle section of thepalladium-supported catalyst in Comparative Example 2 using an X-raymicroprobe.

FIG. 3 shows the results of line analysis of the particle section of thepalladium-supported catalyst in Example 2 using an X-ray microprobe.

BEST MODE FOR CARRYING OUT THE INVENTION

This invention relates to a noble metal-supported article in whichpalladium is supported only in a specific region of a carrier in orderto efficiently and effectively utilize palladium and which article has alayer in which substantially no palladium is supported in the interiorof the carrier. Moreover, it relates to a noble metal-supported articlein which palladium is supported on a carrier in a manner that palladiumis distributed and supported only in a specific region of the carrierand which article has layers in which no palladium is supported in boththe outermost portion and the interior of the carrier. Hereinafter, thelayer in which substantially no palladium is supported is referred to asthe palladium-unsupported layer or merely as the unsupported layer insome cases. Furthermore, the layer in which palladium is supported ishereinafter referred to as the palladium-supported layer or merely asthe supported layer in some cases.

As to the thickness of the palladium that is supported and the thicknessof the layer in which no palladium is supported, the optimum ranges areselected depending upon the thickness or particle diameter of thecarrier. However, it is possible to separate the case where thethickness or particle diameter of the carrier is not less than 200 μmfrom the case where the thickness or particle diameter of the carrier isless than 200 μm and to define the pertinent range in each of thesecases.

For example, a large carrier having a particle diameter of not less than200 μm, for example, several millimeters or more is generally used in aliquid phase reaction in which the reaction is relatively slow or in agas phase reaction. Accordingly, a catalyst which is hardly affected bythe diffusion rate of the reactants can be prepared by supporting acatalyst component participating in the reaction such as palladium orthe like in the range of from the surface to a depth of less than 100 μmof carrier and providing a layer in which no palladium is supported inthe interior of the carrier. Consequently, it follows that palladium canbe efficiently utilized.

On the other hand, when small particles having a particle diameter ofless than 200 μm are used as a carrier, particularly when the noblemetal-supported article obtained is used in a liquid phase reaction, theinfluence of reaction rate and diffusion rate of reactant in the finepores in the interior of the carrier appears, so that such a design hasheretofore been made that the particle diameter is made small accordingto the reaction. In this invention, by making the palladium-supportedlayer thin, a highly active catalyst can be obtained without making theparticle diameter small. That is to say, this invention is advantageousin that the separation of catalyst by settling becomes easy and theseparation by use of a separator having a small volume is made possible.However, when the volume of the portion in which no palladium issupported in the supported article becomes too large, the volumeunnecessary for the reaction per reactor becomes large and a uselessportion is caused in the supported article in some cases. Therefore, itis preferable to set the particle diameter of carrier according to thetype of reaction and set the necessary thickness of thepalladium-supported layer and the necessary thickness of the layer inwhich no palladium is supported.

In the case of a carrier having a particle diameter of less than 200 μm,it is preferable to distribute palladium in the region from the surfaceto a depth of 80 μm of carrier. Moreover, when a layer in whichsubstantially no palladium is supported is provided in the region fromthe outer surface to the specified depth of carrier, it is preferable toprovide a layer in which substantially no palladium is supported in theregion from the outer surface to a depth of 30 μm of carrier and to forma layer in which palladium is supported at a depth of 30 to 80 μm fromthe surface. When abrasion is highly possible and the diffusion rate inthe interior of the carrier is small, it is preferable in some cases toprovide a layer in which substantially no palladium is supported in theregion from the outer surface to a depth of 10 μm of carrier and to forma layer in which palladium is supported at a depth of 10 to 25 μm fromthe surface. On the other hand, when there is no abrasion but theinfluence of the diffusion rate in the fine pores is a problem, thepalladium component can be effectively used by supporting palladium in anarrower region of 10 μm in width in the vicinity of the carriersurface.

When the particle diameter of the carrier is made smaller, namely notmore than 10 μm, the influence of diffusion is lessened and hence theeffect of this invention becomes small in some cases. However, since itis possible to provide the unsupported layer as the outermost layer ofnot more than 1 μm in thickness, a great improvement effect ondurability can be expected. It is preferable to set, in the carrier, thepalladium-supported layer, the layer in which substantially no palladiumis supported in the region from the outer surface to the specified depthand the unsupported layer in the interior based on the purpose andreaction characteristics, select the optimum construction thereof andthen utilize the resulting palladium-supported article.

In this invention, the term “substantially no palladium is supported”means that substantially no peak showing a palladium distribution of arelative intensity of not less than 10% is present in the conventionalX-ray microprobe analytical method as stated hereinafter or in thesecondary electron reflection image of a scanning type electronmicroscope of a high resolving power.

In this invention, carriers having such various sizes that thesubstantial thickness or particle diameter is in the order of from μm tomm or cm and having various shapes can be utilized. Specific examples ofthe carrier shape include spheres, ellipsoids, columns, tablets, hollowcolumns, plates, bars, sheets, honeycombs and the like. The carriershape can be appropriately varied depending upon the reaction type, andin, for example, a fixed bed reaction, a shape which gives only a smallpressure loss such as a hollow column, honeycomb or the like isselected, and under liquid phase slurry suspension conditions, aspherical shape is generally selected.

The kind of carrier used in this invention is not critical as far as itis a conventional carrier such as silica gel, alumina, activated carbon,silica-alumina, silica-alumina-magnesia, zeolite, crystallinemetasilicate or the like or a porous material having pores in the orderof a nanometer. Generally, when a large surface area of carrier isrequired, for example, activated carbon is used, while when a mechanicalstrength is required, for example, alumina, silica-alumina,silica-alumina-magnesia, zeolite, crystalline metasilicate and the likeare used. Thus, the noble metal-supported article of this invention canbe produced by varying the shape and kind of carrier according to thepurpose of utilization.

In this invention, a basic metal salt component of at least one metalselected from the group consisting of alkali metals such as Li, Na, K,Rb, Cs and the like; alkaline earth metals such as Be, Mg, Ca, Sr, Baand the like and rare earth metals such as La, Ce, Pr and the like ispreviously supported on a carrier. As the basic metal salt component,those which are converted into oxides by an operation such ascalcination or the like, for example, nitric acid salts, acetic acidsalts and the like are preferred.

The carrier used in this invention is obtained, for example, bysupporting a soluble salt of at least one metal selected from the groupconsisting of alkali metals, alkaline earth metals and rare earth metalson a carrier such as silica gel, alumina, activated carbon,silica-alumina, zeolite, crystalline metasilicate or the like byimpregnation, adsorption or the like, and then subjecting the same todrying and calcination. Also, a silica gel having an enhanced degree ofdispersion of at least one metal selected from the group consisting ofalkali metals, alkaline earth metals and rare earth metals can beobtained by uniformly dispersing a soluble salt of at least one metalselected from the group consisting of alkali metals, alkaline earthmetals and rare earth metals in a silica sol solution and spray-dryingthe resulting dispersion, and then calcining the dried dispersion.Silica-alumina in which at least one metal selected from the groupconsisting of alkali metals, alkaline earth metals or rare earth metalsare dispersed can be obtained by dispersing a soluble salt of at leastone metal selected from the group consisting of alkali metals, alkalineearth metals and rare earth metals in a silica and alumina sol solution,spray-drying the resulting dispersion and then calcining the drieddispersion. Silica-alumina in which at least one metal selected from thegroup consisting of alkali metals, alkaline earth metals and rare earthmetals is dispersed can be obtained by dispersing a soluble salt of atleast one metal selected from the group consisting of alkali metals,alkaline earth metals and rare earth metals in a silica-alumina sol,spray-drying the resulting dispersion and then calcining the drieddispersion. Silicaalumina-magnesia containing at least one metalselected from the group consisting of alkali metals, alkaline earthmetals and rare earth metals can be obtained by dissolving anddispersing soluble salts of magnesium and aluminum such as magnesiumnitrate, aluminum nitrate and the like in a silica sol solution, furtheruniformly dispersing a soluble salt of at least one metal selected fromthe group consisting of alkali metals, alkaline earth metals and rareearth metals therein, thereafter spray-drying the resulting dispersionand then calcining the dried dispersion.

The drying of the carrier on which a basic metal salt component of atleast one metal selected from the group consisting of alkali metals,alkaline earth metals and rare earth metals has been supported isconducted at a temperature of not less than 100° C., preferably not lessthan 110° C., for 1-48 hours. The calcination is conducted at atemperature of 300-800° C., preferably 300-700° C., more preferably 450°C., for 1-48 hours.

In the production of the noble metal-supported article of thisinvention, any carrier obtained by previously supporting thereon a basicmetal salt component of at least one metal selected from theabove-mentioned alkali metals, alkaline earth metals and rare earthmetals and drying and/or calcining the same can be used.

The palladium compound used in this invention includes soluble salts ofpalladium such as palladium chloride, palladium acetate and the like,and palladium chloride is particularly preferred.

Since palladium chloride is not completely dissolved in water, it isusual to sufficiently dissolve palladium in an aqueous hydrochloric acidsolution or an aqueous sodium salt solution to prepare a palladiumchloride solution.

As the aluminum compound solution used in this invention, an aqueoussolution of a soluble aluminum salt such as aluminum chloride, aluminumnitrate or the like is used.

The production process of this invention is based on the principle thatthe palladium component is insolubilized and immobilized by a chemicalreaction between the soluble palladium compound and the basic metal saltcomponent of at least one metal selected from the group consisting ofalkali metals, alkaline earth metals and rare earth metals previouslysupported on a carrier.

Moreover, the production process of this invention in which apalladium-unsupported layer is formed as the outermost layer is alsobased on the above-mentioned principle of insolubilization andimmobilization. First of all, as the aluminum compound, a solublealuminum salt such as aluminum chloride, aluminum nitrate or the like isused, and aluminum is allowed to react with the carrier on its outersurface through a chemical reaction between the soluble aluminum saltand a basic metal salt component of at least one metal selected from thegroup consisting of alkali metals, alkaline earth metals and rare earthmetals, thereby consuming the reaction sites of palladium, and the basicmetal salt component of at least one metal selected from the groupconsisting of alkali metals, alkaline earth metals and rare earth metalsin the interior and the palladium component in the interior areimmobilized by reaction.

The amount of the aluminum component is varied depending upon thethickness of the layer in which the palladium-containing metal componentis not supported and determined based on the amount of the basic metalsalt component of at least one metal selected from the group consistingof alkali metals, alkaline earth metals and rare earth metals previouslysupported on the carrier. Usually, it is 0.001 to 2 moles, preferably0.005 to 1 mole, per mole of the basic metal salt component of at leastone metal selected from the group consisting of alkali metals, alkalineearth metals and rare earth metals previously supported on the carrier.

There are many unclear points on the details of the mechanism by whichthe palladium distribution of this invention is achieved; however, thepresent inventors have inferred that the diffusion rate of thepalladium-containing soluble component in the carrier interior and theinsolubilization rate of said component by a chemical reaction are wellbalanced, whereby it has been made possible to immobilize palladium in avery narrow region in the vicinity of the carrier surface. Moreover,when the carrier has on its outer surface a layer in which substantiallyno palladium is present, the present inventors have inferred that whenaluminum is allowed to react with the basic metal salt component in thevicinity of the outer surface of the carrier to consume the basic metalsalt component capable of reacting with the palladium component in thevicinity of the outer surface of the carrier, and subsequently,palladium is supported, palladium reacts with the basic metal saltcomponent in the carrier interior because the reactive basic metal saltcomponent in the vicinity of the outer surface of the carrier hasalready been consumed, whereby the palladium is immobilized in thecarrier interior.

The noble metal-supported article of this invention can be obtained bymomentarily introducing a carrier obtained by previously dispersingtherein a basic metal salt component of at least one metal selected fromthe group consisting of alkali metals, alkaline earth metals and rareearth metals and subjecting the carrier to drying and/or calcination,into a solution of a palladium compound such as palladium chloride orthe like maintained at a temperature of not less than 70° C.

The noble metal-supported article of this invention which also has theunsupported layer as the outermost layer can be obtained by instantlyintroducing a carrier obtained by subjecting a basic metal saltcomponent of at least one metal selected from the group consisting ofalkali metals, alkaline earth metals and rare earth metals to previousdispersion and then to drying and/or calcination, into an aluminumsolution maintained at not less than 70° C. to allow the carrier toreact with the aluminum to form a surface layer in which substantiallyno palladium is supported, and subsequently contacting a solution of apalladium compound such as palladium chloride or the like with thecarrier at not less than 70° C. The supported article thus obtained hasthe palladium-unsupported layer as the outermost layer and the thicknessof the supported layer is less than 100 μm.

The production process of this invention utilizes immobilization due tochemical reaction and the reaction is completed in a short time, andhence, is an epoch-making process which, unlike support obtained bymeans of ion-exchange, does not require the carrier introduced into areactant solution to be taken out from the reaction mixture in a shorttime after the introduction, enables the palladium-supported layer andthe palladium-unsupported layer to be sharply controlled with a goodprecision, and enables palladium to be supported on a carrier in amanner that the palladium distribution deviates in the direction of thevicinity of the surface. Upon introducing the carrier into the palladiumcompound solution or aluminum compound solution, this compound reactswith such a component as an alkali metal or the like in the vicinity ofthe carrier surface so as to be insolubilized and immobilized, wherebythe palladium-supported layer and the palladium-unsupported layer areformed. Accordingly, when this insolubilization-immobilization reactionis slow, the palladium or aluminum component diffuses in the fine poresof the carrier to its center portion and it follows that thepalladium-supported layer or palladium-unsupported layer on the surfaceis broadened.

That is to say, in one mode of the production process of this invention,each of the following conditions is satisfied: (i) The temperature ofthe palladium compound solution is maintained at not less than 70° C.(ii) A carrier having previously dispersed therein a basic metal saltcomponent of at least one metal selected from the group consisting ofalkali metals, alkaline earth metals and rare earth metals is subjectedto drying and/or calcination and then used. (iii) The dried and/orcalcined carrier is added to a palladium compound solution. (iv) Thecarrier is instantly added to a palladium compound solution.

Moreover, when the palladium-unsupported layer is provided as theoutermost layer, each of the following conditions is satisfied: (i) Thetemperature of the aluminum compound solution is maintained at not lessthan 70° C. (ii) A carrier having previously dispersed therein a basicmetal salt component of at least one metal selected from the groupconsisting of alkali metals, alkaline earth metals and rare earth metalsis subjected to drying and/or calcination and then used. (iii) The driedand/or calcined carrier is added to an aluminum compound solution.(iv) 1) The carrier is momentarily added to an aluminum compoundsolution, and then a palladium compound solution maintained at atemperature of not less than 70° C. is momentarily added thereto, or 2)the carrier is momentarily added to a solution of a mixture of analuminum compound and a palladium compound.

The reaction temperature in (i) above is one of the control factors andis varied depending upon the amount of the basic metal salt component ofat least one metal selected from the group consisting of alkali metals,alkaline earth metals and rare earth metals previously supported on thecarrier; however, when the temperature is too low, the reaction becomesslow and the metal distribution is broadened. In the production of thenoble metal-supported article of this invention, the temperature ispreferably so high that a high reaction rate is obtained in order toobtain a sharp layer. It is important that the temperature is not lessthan 70° C., and the temperature is preferably not less than 80° C.,more preferably not less than 90° C. The reaction can be effected at atemperature not lower than the boiling point of the solution underpressure, but it is usually preferable to conduct the reaction at atemperature not higher than the boiling point for facilitating theoperation.

It is necessary to use a carrier in which a basic metal salt componentof at least one metal selected from the group consisting of alkalimetals, alkaline earth metals and rare earth metals has previously beendispersed. In the production process of this invention, by theinsolubilization-immobilization reaction between the basic metal saltcomponent and aluminum and palladium, the palladium-unsupported layer isprovided on the carrier surface and palladium is immobilized in theinterior of the carrier. Furthermore, the carrier must previously besubjected to drying and/or calcination before its use. The fine pores ofthe carrier which have been subjected to drying and/or calcination arefilled with a gas component such as air or the like and this gascomponent prevents the palladium compound solution and aluminum compoundsolution from diffusing to the center portion of the carrier through thefine pores. This is believed to be one of the factors for making thedistribution sharp.

As in (iii) above, it is necessary to add the dried and/or calcinedcarrier to the palladium compound solution or the aluminum compoundsolution. By previously dispersing the carrier in only water or the likeand then adding thereto the palladium compound solution or the aluminumcompound solution, it is also possible to obtain the noblemetal-supported article having a relatively sharp distribution of thepalladium-supported layer and the palladium-unsupported layer as far asthe above (i), (ii) and (iv) conditions are satisfied. However, a partof the base component in the carrier is dissolved in the liquid in somecases, whereby the deterioration of liquid properties and thedeterioration of operability are caused in some cases. Therefore, suchan addition method is not desirable.

As stated in (iv) above, the introduction of the carrier is effectedinstantly. When the introduction of the carrier is conducted over a longperiod of time, it follows that there is obtained a mixture of carriershaving various distribution states which are probably due to the timehistory needed for the introduction. In this invention, the term“instantly” means that the time needed for introducing the carrier iswithin 15 seconds, preferably within 10 seconds though it may be varieddepending upon the thickness and particle diameter of the carrierintroduced. It is more preferable to bring the time of introduction asclose to zero seconds as is possible.

Another example of a production process of this invention in whichaluminum or both aluminum and a palladium solution are added to acarrier slurry is shown below.

In this mode of the production process of the invention, each of thefollowing conditions is satisfied: (i) A solution of a salt of at leastone metal selected from the group consisting of alkali metals, alkalineearth metals and rare earth metals is maintained at a temperature of notless than 70° C. (ii) A dried and/or calcined carrier in which a basicmetal salt component of at least one metal selected from the groupconsisting of alkali metals, alkaline earth metals and rare earth metalshas previously been dispersed is instantly added to the solutionmentioned in (i) above to prepare a slurry. (iii) A palladium compoundsolution is instantly added to the slurry.

Furthermore, the palladium-supported article having thepalladium-unsupported layer as the outermost layer can be obtained by(i) maintaining a solution of a salt of at least one metal selected fromthe group consisting of alkali metals, alkaline earth metals and rareearth metals at a temperature of not less than 70° C., (ii) instantlyadding to the solution of (i) above a dried and/or calcined carrier inwhich a basic metal salt component of at least one metal selected fromthe group consisting of alkali metals, alkaline earth metals and rareearth metals has previously been dispersed to prepare a slurry, andthereafter (iii) momentarily adding to the slurry an aluminum compoundsolution and then a palladium compound solution or a mixed solution ofthe aluminum compound solution and the palladium compound solution.

As the salt of at least one metal selected from the group consisting ofalkali metals, alkaline earth metals and rare earth metals used in theabove step (i), there is used at least one member selected from thegroup consisting of organic acid salts, inorganic acid salts, hydroxidesand the like of the above metals; however, acetic acid salts and nitricacid salts which are converted into oxides upon calcination arepreferable.

In addition, since in the noble metal-supported article of thisinvention, an aluminum ion and a palladium ion are allowed to react withat least one metal selected from the group consisting of alkali metals,alkaline earth metals and rare earth metals, the amount of the basicmetal salt component of at least one metal selected from the groupconsisting of alkali metals, alkaline earth metals and rare earth metalsdispersed and immobilized in the carrier is varied depending upon thethickness of the palladium-unsupported layer on the surface of thecarrier or the amount of palladium to be supported on the carrier;however, it is selected from the range of from 1 to 100 moles,preferably from 2 to 50 moles, per mole of the palladium supported.

In the production process of this invention, the amount of the carrierused is usually 5 to 200 times the amount of palladium supported.

Next, a method of preparing a metallic palladium-supported article bysubjecting a supported article in which a palladium compound isimmobilized in a reduction operation is described. The metallicpalladium-supported article can be obtained by dispersing the supportedarticle in which a palladium component is dispersed and immobilized, inan aqueous solution or the like, and then subjecting the resultingdispersion with stirring to reduction treatment using formalin, formicacid, hydrazine, methanol, hydrogen gas or the like.

As described above, according to the process of this invention, it has,for the first time, become possible to obtain a palladium-supportedarticle which has a palladium-supported layer in a specific shallowregion in the vicinity of the outer surface of the carrier and whichfurther has a layer in which no palladium is supported on the outersurface of the carrier.

Furthermore, according to the process of this invention, there can beproduced a noble metal-supported article in which a palladiumintermetallic compound in which the lattice of palladium has beenreplaced with a diverse metal or a palladium alloy in which palladiumand a diverse metal have formed a solid solution is supported. In thisinvention, the palladium intermetallic compound and the palladium alloyare the palladium metal compounds as defined hereinbefore.

This invention is now explained below using as an example a process forproducing a palladium-lead intermetallic compound effective as acatalyst to be used in the production of a carboxylic acid ester from analdehyde and an alcohol by an oxidative carbonylation reaction in thepresence of oxygen.

First of all, a soluble palladium compound such as palladium chloride orthe like is insolubilized and immobilized on a carrier by theabove-mentioned process for producing a noble metal-supported article ofthis invention. In this stage, the distribution of the palladiumcomponent has been controlled. Subsequently, using a solution of asoluble lead salt such as lead nitrate or the like, the lead componentis supported on the carrier. By the subsequent reduction treatment, apalladium-lead intermetallic compound-supported article can be obtained.Moreover, the very high purity palladium-lead intermetallic compoundcatalyst of the invention exhibiting excellent performance in thecarboxylic ester producing reaction can be effectively obtained bysubjecting the supported palladium ions and lead ions to reductiontreatment in the presence of both lead ions and at least one compoundselected from the group consisting of C₁ to C₅ fatty acids, alkali metalsalts and alkaline earth metal salts.

The principle of the production process of this invention is applied topalladium-lead intermetallic compounds comprising palladium and lead anda minor amount of at least one other diverse element, for example,mercury, thallium, bismuth, tellurium, nickel, chromium, cobalt, indium,tantalum, copper, zinc, zirconium, hafnium, tungsten, manganese, silver,rhenium, antimony, tin, rhodium, ruthenium, iridium, platinum, gold,titanium, aluminum, boron, silicon or the like, and according to amethod based on the above principle, there can be obtained a palladiummetal compound-supported article in which the metal distribution in thecarrier has been controlled.

As the compounds of lead and mercury, thallium, bismuth, tellurium,nickel, chromium, cobalt, indium, tantalum, copper, zinc, zirconium,hafnium, tungsten, manganese, silver, rhenium, antimony, tin, rhodium,ruthenium, iridium, platinum, gold, titanium, aluminum, boron orsilicon, there are mentioned organic acid salts, organic complexes,inorganic acid salts, hydroxides and the like. As the lead compound,there are suitable, for example, lead nitrate, lead acetate and thelike.

Even in the palladium-lead intermetallic compound, the amount ofpalladium supported is not critical; however, it is usually 0.1 to 20%by weight, preferably 1 to 10% by weight, based on the weight of thecarrier. The amount of lead supported is also not critical, and it isusually 0 to 20% by weight, preferably 1 to 10% by weight, based on theweight of the carrier.

In the case of a palladium-lead intermetallic compound which isparticularly effective as a catalyst for obtaining a carboxylic acidester from an aldehyde and an alcohol, important is the atomic ratio ofthe supported palladium and lead rather than the individual amounts ofsupported palladium and lead. That is to say, in the case of apalladium-lead intermetallic compound, the Pd/Pb atomic ratio of thecatalyst obtained by supporting palladium and lead on a carrier isselected from the range of from 3/0.1 to 3/3, preferably from 3/0.1 to3/2.0 and more preferably from 3/0.8 to 3/1.5.

As the C₁-C₅ fatty acid to be added in the reduction treatment step forforming the palladium-lead intermetallic compound, there can be usedpropionic acid, acetic acid, butyric acid, maleic acid, acrylic acid,methacrylic acid and the like; however, easily available acetic acid ispreferred. The C₁-C₅ fatty acid can be added in an amount of preferably0.1 to 30 moles, more preferably 1 to 15 moles, per mole of thesupported palladium of the catalyst, and can be used within the range ofthe solubilities of lower fatty acids.

As the alkali or alkaline earth metal to be added in the reductiontreatment step for forming the palladium-lead intermetallic compound,there can be used either hydroxides or fatty acid salts; however, fattyacid salts are preferred in view of operability and the like.

The reduction treatment for forming the palladium-lead intermetalliccompound-supported article of this invention can be conducted at atemperature of room temperature to 200° C. When the temperature is notlower than the boiling point, the necessary pressure for keeping theliquid phase is applied. Preferably, the reduction is carried out underthe conditions of room temperature to 160° C. and normal pressure toseveral atmospheres. Moreover, the time for the reduction treatment maybe varied depending upon the treatment conditions; however, it isgenerally 0.5 to 5 hours. Usually, it is preferable to set theconditions so that the treatment is completed within 24 hours in view ofthe operability.

In the process of this invention, in addition to the palladium metalcompound containing lead, there can be similarly used the palladiummetal compounds containing bismuth, thallium, mercury, tellurium,copper, gold or the like as a metal species, thereby obtaining thepalladium metal compound-supported article in which the distribution ofthese metals has been controlled on a carrier.

The metallic palladium-supported article and palladium metalcompound-supported article obtained by the process of this invention canbe broadly used as catalysts in catalytic reactions. They can be used ascatalysts for such reactions as, for example, oxidative carboxylicester-producing reactions between aldehydes and alcohols, partialhydrogenation of acetylenes to olefins, complete hydrogenation ofacetylenes to paraffins, conversion of diolefins to monoolefins,selective hydrogenation of olefins, aliphatic dehalogenation, aromaticdehalogenation, reduction of acid chlorides, hydrogenation of aromaticnitro compounds to aromatic amines, hydrogenation of aromatic carbonyls,ring hydrogenation of benzoic acid, hydrogenation of phenol tocyclohexane, hydrogenation of aromatic ketones to aromatic alcohols,hydrogenation of aromatic ketones to alkyl aromatics, hydrogenolysis ofaromatic carbonyls, decarbonylation of aromatic carbonyls, hydrogenationof aromatic nitrites to aromatic amines, hydrogenation of aromaticnitrites to aromatic aldehydes, disproportionation of cylohexene,transfer of olefins, reductive N-methylation of anilines, hydrogenationof aromatic nitro compounds to hydrazobenzene compounds, hydrogenationof nitrohexanes to cyclooxanones, hydrogenation of nitro-olefins toalkylamines, hydrogenation of oxams to primary amines, debenzylation,hydrogenation of epoxides to alcohols, reductive amination,hydrogenation of quinones to hydroquinones, ring hydrogenation ofaromatic esters, hydrogenation of furan rings, ring hydrogenation ofpyridine compounds, hydrogenation of nitric acid salts tohydroxylamines, hydrogenation of peroxides, hydrogenation of aliphaticnitro compounds, acetoxylation, carbonylation, dehydrogenation,dehydrogenation, liquid phase oxidation, deoxo reactions, oxidation ofcarbon monoxide, reduction of NO_(x) and the like.

This invention is specifically explained below by way of Examples andComparative Examples.

Measurement of palladium distribution by EPMA

A sample obtained by embedding the noble metal-supported articleobtained in a resin and polishing the same was subjected to lineanalysis in the depth direction of a particle section under thefollowing conditions using an X-ray microprobe (EPMA) (JXA-8800R, atrade name of JEOL Ltd.):

Accelerating voltage: 15 kV

Scan method: Stage scan

Measurement step space: 0.2-0.3 μm

Measurement time/step: 150 msec

Setting of electron beam diameter: 0 μm

Electron beam current value: 2×10−8A

Spectral crystal: Pd=PETH, Pb=PETH, Si=TAP

Carrier Production Reference Example

Aluminum nitrate and magnesium nitrate were added to SNOW TEX N-30 (SiO₂content: 30% by weight) (a trade name of Nissan Chemical Industries,Ltd.) as a silica sol so that the Al/(Si+Al) proportion became 10 mole %and the Mg/(Si+Mg) proportion became 10 mole %, respectively, anddissolved therein, and thereafter, the resulting solution wasspray-dried in a spray drier set at a temperature of 130° C. to obtain aspherical carrier having an average particle diameter of 60 μm. Thisspherical carrier was calcined in air at 300° C. for 2 hours and then at600° C. for 3 hours. This was used as the carrier in this invention.

EXAMPLE 1

Palladium chloride was added to an aqueous solution of 15% by weight ofpalladium chloride and 10% by weight of sodium chloride with stirring sothat the proportion of palladium became 5 parts by weight per 100 partsby weight of a carrier to completely dissolve the palladium chloride inthe solution, and the temperature thereof was maintained at 90° C.Subsequently, the carrier in the dry state obtained in the CarrierProduction Reference Example was instantly introduced into the abovesolution. Subsequently, the supernatant was decanted and the carrier onwhich palladium had been adsorbed was washed with distilled waterseveral times. Distilled water was added to the washed carrier and tothe resulting mixture which was being stirred at 90° C. was dropwiseadded an aqueous hydrazine solution with stirring in a proportion of 3moles per mole of Pd over about 30 minutes. The resulting mixture wassubjected to reduction treatment for 24 hours to obtain apalladium-supported catalyst.

The catalyst thus obtained was embedded in a resin and polished toprepare a sample and this sample was subjected to line analysis of aparticle section using an X-ray microprobe (EPMA) (JXA-8800R, a tradename of JEOL Ltd.). The results obtained are shown in FIG. 1. As isclear from FIG. 1, it was confirmed that Pd was distributed andsupported in the vicinity of the surface, namely, in the depth region ofnot more than 15 microns and was not present in the interior.

COMPARATIVE EXAMPLE 1

The same procedure as in Example 1 was repeated, except that thetemperature for allowing palladium to be adsorbed was changed to roomtemperature, to obtain a palladium-supported catalyst.

The EPMA analysis results of the catalyst obtained are shown in FIG. 2.From FIG. 2, it was seen that Pd was uniformly distributed and supportedfrom the surface to the interior of the carrier.

EXAMPLE 2

An aqueous aluminum nitrate solution in an amount corresponding to 0.35part by weight of aluminum per 100 parts by weight of the carrier ofExample 1 was heated to 90° C. and stirred, and the carrier in the drystate obtained in the Carrier Production Reference Example was instantlyadded thereto. The resulting mixture was stirred for a further 15minutes at 90° C. Subsequently, to the mixture was instantly added anaqueous solution of 15% by weight of palladium chloride and 10% byweight of sodium chloride, which solution was heated to 90° C., in suchan amount that the proportion of palladium became 5 parts by weight per100 parts by weight of the carrier and the resulting mixture was stirredfor a further one hour at 90° C. to allow the palladium chloride to becompletely adsorbed on the carrier. Subsequently, the liquid wasdecanted and the carrier on which palladium had been adsorbed was washedwith distilled water several times. Subsequently, the carrier thusobtained was introduced into a 18% by weight aqueous sodium acetatesolution in such a proportion that the amount of sodium acetate became18 times the amount of the palladium and the resulting mixture wasstirred. Thereafter, lead acetate was added thereto in such an amountthat Pd/Pb became 3/1.3 and the resulting mixture was stirred at 90° C.for about 30 minutes. Subsequently, an aqueous hydrazine solution wasgradually dropwise added with stirring to the resulting mixture in suchan amount that the proportion of hydrazine became 3 moles per mole ofthe total of Pd and Pb, and the resulting mixture was subjected toreduction treatment for 24 hours.

The catalyst thus obtained was analyzed by the above-mentioned method tofind that it was a palladium-lead intermetallic compound whose X-raydiffraction angle (2 θ) at the maximum intensity peak in the powderX-ray diffraction pattern was 38.625°.

The catalyst obtained was embedded in a resin and polished to obtainparticles, and the particle thus obtained was subjected to line analysisof a particle section using an X-ray microprobe (EPMA) (JXA-8800R, atrade name of JEOL Ltd.). The results obtained are shown in FIG. 3. Asis clear from FIG. 3, it was confirmed that Pd was not present in theregion from the surface to a depth of 2 microns, was supported in thedepth region of not less than 15 microns and was not present in theinterior.

EXAMPLE 3

In a stirring type stainless steel reactor having a 1.2-liter liquidphase portion and provided with a catalyst separator was placed 200 g ofthe catalyst of Example 2, and an oxidative carboxylic ester-producingreaction between an aldehyde and an alcohol was carried out while thecontents were stirred at such a rate that the tip speed of the stirringblade was 4 m/s. A 36.7% by weight methanolic methacrolein solution towhich lead acetate had been added in such an amount that theconcentration thereof in the reaction system became 30 ppm and a 2-4% byweight methanolic NaOH solution were continuously fed at 0.54 liter/hrand 0.06 liter/hr, respectively, to the reactor (the methacroleinconcentration in the reaction system consisting of the above twosolutions was about 33% by weight); air was blown thereinto so that theoxygen partial pressure at the outlet of the reactor became 0.2 atm. ata reaction temperature of 80° C. and at a reaction pressure of 5 kg/cm²;and the concentration of NaOH fed to the reactor was controlled so thatthe pH of the reaction system became 7.1. The reaction product wascontinuously withdrawn by overflow from the outlet of the reactor andsubjected to comparative evaluation by gas chromatography. After a lapseof 20 hours, the selectivity of methyl methacrylate (MMA) which was theobjective product was 91.4% and the MMA production rate was 5.02moles/hr/Kg-catalyst. After the reaction had been effected for 2,000hours, the selectivity of methyl methacrylate (MMA) which was theobjective product was 91.8% and the MMA production rate was 5.11moles/hr/Kg-catalyst. Comparing the amount of Pd supported after 2,000hours with the initial amount of Pd supported, it was found that theloss of palladium was 0.1% or less.

COMPARATIVE EXAMPLE 2

The reaction was conducted under the same operation conditions as inExample 3, except that the catalyst of Example 2 was changed to thecatalyst of Comparative Example 1. After a lapse of 20 hours, theselectivity of methyl methacrylate (MMA) which was the objective productwas 91.1% and the MMA production rate was 4.32 moles/hr/Kg-catalyst. Theselectivity of methyl methacrylate (MMA) which was the objective productafter the reaction had been effected for 2,000 hours was 91.3% and theMMA production rate was 3.89 moles/hr/Kg-catalyst. The catalyst wasrecovered and the amount of Pd supported after 2,000 hours was comparedwith the initial amount of Pd supported to find that the loss ofpalladium was 5.2%.

EXAMPLE 4

100 parts by weight of a silica gel carrier manufactured by Fuji Silicia(CARIACT-10, 150 μm in diameter) was introduced into an aqueouspotassium acetate solution containing 5 parts by weight of potassium,and the resulting mixture was subjected to impregnation and drying. Themixture was further calcined at 600° C. for 3 hours in air.Subsequently, an aqueous aluminum nitrate solution containing 0.6 moleof aluminum per mole of palladium was heated to 90° C. and stirred, intowhich the carrier was momentarily introduced. The resulting mixture wasstirred at 90° C. for a further 15 minutes. The temperature of themixture was lowered to room temperature, at which temperature an aqueoussolution of 15% by weight of palladium chloride and 10% by weight ofsodium chloride was added to the mixture with stirring in such an amountthat the proportion of palladium became 5 parts by weight per 100 partsby weight of the carrier, whereby palladium chloride was allowed to becompletely adsorbed on the carrier. Subsequently, the liquid wasdecanted and the carrier on which palladium had been adsorbed was washedwith distilled water several times. Distilled water was added theretoand the resulting mixture was stirred at 80° C., and to the mixture inthis state was gradually dropwise added an aqueous hydrazine solution insuch an amount that the proportion of hydrazine became 3 moles per moleof Pd with stirring over about 30 minutes and the resulting mixture wassubjected to reduction treatment for 24 hours to obtain apalladium-supported catalyst.

The catalyst thus obtained was embedded in a resin and polished toobtain a sample, and this sample was subjected to line analysis of aparticle section using an X-ray microprobe (EPMA) (JXA-8800R, a tradename of JEOL Ltd.) to determine the distribution of palladium, fromwhich it was found that the carrier had a palladium-unsupported layer inthe region from the surface to a depth of 5 microns, and palladium wassupported in the depth region of 30 microns and was not present in theinterior.

EXAMPLE 5

100 parts by weight of a spherical alumina carrier manufactured bySumitomo Chemical Co., Ltd. (KHD, 3 mm in diameter) was introduced intoan aqueous potassium nitrate solution containing 4 parts by weight ofpotassium and the resulting mixture was subjected to impregnation anddrying, and further to calcination at 600° C. for 3 hours in air.Subsequently, an aqueous solution of 15% by weight of palladium chlorideand 10% by weight of sodium chloride was stirred at 70° C. and thecarrier was instantly added thereto in such an amount that theproportion of palladium became 5 parts by weight per 100 parts by weightof the carrier, after which the resulting mixture was stirred for afurther one hour at 70° C. Subsequently, the liquid was decanted, andthe carrier on which palladium had been adsorbed was washed withdistilled water several times. Distilled water was added to the carrierand the resulting mixture was stirred at 70° C. To the mixture in thisstate was gradually dropwise added an aqueous hydrazine solution in suchan amount that the proportion of hydrazine became 3 moles per mole of Pdwith stirring over about 30 minutes and the resulting mixture wassubjected to reduction treatment for 24 hours to obtain apalladium-supported catalyst.

The catalyst thus obtained was embedded in a resin and polished toprepare a sample, and this sample was subjected to line analysis of aparticle section using an X-ray microprobe (EPMA) (JXA-8800R, a tradename of JEOL Ltd.) to determine the distribution of palladium, fromwhich it was found that in the carrier, palladium was supported in theregion from the surface to a depth of 50 microns and was not present inthe interior.

EXAMPLE 6

An aqueous solution having dissolved therein magnesium chloride in anamount equimolar to palladium was heated to 90° C. and stirred. Intothis aqueous solution was instantly introduced the carrier in the drystate obtained in the Carrier Production Reference Example, and theresulting mixture was stirred at 90° C. for a further 10 minutes. Theresulting carrier slurry maintained at a temperature of 90° C. was addedwith stirring to an aqueous solution of a mixture of 15% by weight ofpalladium chloride and 10% by weight of sodium chloride which solutioncontained 0.2 mole of aluminum per mole of palladium, in such aproportion that the amount of palladium became 3 parts by weight per 100parts by weight of the carrier, to allow palladium to be completelyadsorbed on the carrier. Thereafter, the liquid was decanted and thecarrier on which palladium had been adsorbed was washed with distilledwater several times and then 1-propanol was substituted for thedistilled water. After the carrier was heated to 60° C., a 1-propanolsolution of triphenylbismuth was added thereto in such an amount thatthe proportion of bismuth became 2.3 parts by weight per 100 parts byweight of the carrier, after which the resulting mixture was stirred. Tothe mixture in this state was gradually dropwise added a solution ofhydrazine in 1-propanol in such an amount that the proportion ofhydrazine became 3 moles per mole of Pd over about 30 minutes, and theresulting mixture was subjected to reduction treatment for 24 hours toobtain a palladium-bismuth-supported catalyst.

The catalyst thus obtained was embedded in a resin and polished toprepare a sample. This sample was subjected to line analysis of aparticle section using an X-ray microprobe (EPMA) (JXA-8800R, a tradename of JEOL Ltd.), to find that the carrier had the Pd-unsupportedlayer in the region from the surface to a depth of 3 microns, and Pd wassupported in the region from the surface to a depth of not more than 15microns and was not present in the interior.

EXAMPLE 7

In a stirring type stainless steel reactor having a 1.2-liter liquidphase portion and provided with a catalyst separator was placed 200 g ofthe catalyst of Example 6, and an oxidative carboxylic ester-producingreaction between an aldehyde and an alcohol was carried out while thecontents were stirred at such a rate that the tip speed of the stirringblade was 4 m/s. A 36.7% by weight methanolic acrolein solution to whichlead acetate had been added so that the amount thereof in the reactionsystem became 20 ppm and a 2-4% by weight methanolic NaOH solution werecontinuously fed at 0.54 liter/hr and 0.06 liter/hr, respectively, tothe reactor (the acrolein concentration in the reaction systemconsisting of the above two solutions was about 33% by weight); air wasblown thereinto so that the oxygen partial pressure at the outlet of thereactor became 0.2 atm at a reaction temperature of 80° C. and at areaction pressure of 5 kg/cm²; and the concentration of NaOH fed to thereaction was controlled so that the pH of the reaction system became7.1. The reaction mixture was continuously withdrawn by overflow fromthe outlet of the reactor and subjected to comparative evaluation by gaschromatography. After a lapse of 20 hours, the selectivity of methylacrylate (MA) which was the objective product was 92.2% and the MAproduction rate was 5.92 moles/hr/Kg-catalyst. After the reaction hadbeen effected for 2,000 hours, the selectivity of methyl acrylate (MA)which was the objective product was 92.6% and the MA production rate was5.97 moles/hr/Kg-catalyst. Comparing the amount of Pd supported after2,000 hours with the initial amount of Pd supported, it was found thatthe loss of palladium was 0.1% or less.

INDUSTRIAL APPLICABILITY

A noble metal-supported article having a high reaction activity can beobtained by supporting palladium or a palladium metal compound on acarrier in the vicinity of its surface. Moreover, a noblemetal-supported article capable of inhibiting the loss of a palladiumcomponent due to abrasion or the like can be obtained by providing theoutermost layer free from palladium. When the noble metal component isallowed to be supported on a carrier in the vicinity of its surface, thecatalyst activity becomes high and the loss of the catalyst componentbecomes small as can be seen from the examples of the carboxylicester-producing reaction between an aldehyde and an alcohol set forthabove. Hence, a grate improvement is economics can be obtained not onlywith this specific reaction, as compared with conventional catalysts,but also more broadly with many reactions in general.

What is claimed is:
 1. A noble metal-supported article which comprises acarrier particle having an outer surface and having a thickness orparticle diameter of less than 200 μm and a palladium-containing metalcomponent supported on the carrier, which article has (A) a portion inan interior of the carrier, in which portion substantially no palladiumis supported and (B) a layer in a region from the outer surface to adepth of less than 80 μm from the outer surface, in which layerpalladium is supported.
 2. The noble metal-supported article accordingto claim 1, which has (C) an outermost layer which is at the outersurface of the carrier and in which substantially no palladium issupported, (B) a layer in a region from an inner side of said outermostlayer (C) to a depth of less than 80 μm from the outer surface of thecarrier in which layer palladium is supported, said layer being incontact with said outermost layer (C) and (A) a portion in an interiorof the carrier in which portion substantially no palladium is supported.3. The noble metal-supported article according to claim 1 or 2, whereinthe palladium-containing metal component is palladium or a palladiummetal compound.
 4. The noble metal-supported article according to claim1, wherein the palladium metal compound is a compound in which part ofthe crystal lattice of palladium has been replaced with bismuth.
 5. Thenoble metal-supported article according to claim 1, wherein thepalladium metal compound is a compound in which part of the crystallattice of palladium has been replaced with lead.
 6. A noblemetal-supported article according to claim 2, wherein layer (C) is notpresent.
 7. A process for producing the noble metal-supported articleaccording to claim 6 which comprises instantly introducing a driedand/or calcined carrier obtained by drying and/or calcining a carriercontaining a soluble salt of at least one metal selected from the groupconsisting of alkali metals, alkaline earth metals and rare earth metalsinto a palladium compound solution maintained at a temperature of notless than 70° C. in an excessive amount 5-200 times as much as theamount of palladium supported.
 8. A process for producing the noblemetal-supported article according to claim 1 which comprises instantlyintroducing a dried and/or calcined carrier obtained by drying and/orcalcining a carrier containing a soluble salt of at least one metalselected from the group consisting of alkali metals, alkaline earthmetals and rare earth metals into an aluminum compound solutionmaintained at a temperature of not less than 70° C. in an excess amount5-200 times as much as the amount of palladium supported and thereafterinstantly introducing the resulting mixture into a palladium compoundsolution maintained at a temperature of not less than 70° C., orinstantly introducing the above dried and/or calcined carrier into asolution of a mixture of an aluminum compound and a palladium compoundmaintained at a temperature of not less than 70° C.
 9. A process forproducing the noble metal-supported article according to claim 6, whichcomprises dispersing a dried and/or calcined carrier obtained by dryingand/or calcining a carrier containing a soluble salt of at least onemetal selected from the group consisting of alkali metals, alkalineearth metals and rare earth metals in an excessive amount 5-200 times asmuch of palladium supported in a solution containing a salt of at leastone metal selected from the group consisting of alkali metals, alkalineearth metals and rare earth metals and then contacting the resultingdispersion with a palladium compound solution at a temperature of notless than 70° C.
 10. A process for producing the noble metal-supportedarticle according to claim 2, which comprises dispersing a dried and/orcalcined carrier obtained by drying and/or calcining a carriercontaining a soluble salt of at least one metal selected from the groupconsisting of alkali metals, alkaline earth metals and rare earth metalsin an excessive amount 5-200 times as much as the amount of palladiumsupported in a solution containing a salt of at least one metal selectedfrom the group consisting of alkali metals, alkaline earth metals andrare earth metals and then contacting the resulting dispersion firstlywith an aluminum compound solution at a temperature of not less than 70°C. and secondly with a palladium compound solution at a temperature ofnot less than 70° C., or contacting the above dispersion with a solutionof a mixture of a palladium compound and an aluminum compound at atemperature of not less than 70° C.
 11. A process for producing anacrylic acid ester and/or a methacrylic acid ester, which comprisesreacting an alcohol with acrolein and/or methacrolein in the presence ofoxygen using the noble metal-supported article according to claim 1 or 2as a catalyst.